Aim: We assessed the effects of flexing and massage on human skin penetration and toxicity of topically applied coated and uncoated zinc oxide nanoparticles (˜75 nm) in vivo. Materials & methods: Noninvasive multiphoton tomography with fluorescence lifetime imaging was used to evaluate the penetration of nanoparticles through the skin barrier and cellular apoptosis in the viable epidermis. Results: All nanoparticles applied to skin with flexing and massage were retained in the stratum corneum or skin furrows. No significant penetration into the viable epidermis was seen and no cellular toxicity was detected. Conclusion: Exposure of normal in vivo human skin to these nanoparticles under common in-use conditions of flexing or massage is not associated with significant adverse events.

Background: Treatment of bacterial biofilms are difficult and in many cases, expensive. Bacterial biofilms are naturally more resilient to antimicrobial agents than their free-living planktonic counterparts, rendering the community growth harder to control. The present work described the risks of long-term use of an important alternative antimicrobial, silver nanoparticles (NAg), for the first time, on the dominant mode of bacterial growth. Results: NAg could inhibit the formation as well as eradicating an already grown biofilm of Pseudomonas aeruginosa, a pathogen notorious for its resilience to antibiotics. The biofilm-forming bacterium however, evolved a reduced sensitivity to the nanoparticle. Evidence suggests that survival is linked to the development of persister cells within the population. A similar adaptation was also seen upon prolonged exposures to ionic silver (Ag⁺). The persister population resumed normal growth after subsequent passage in the absence of silver, highlighting the potential risks of recurrent infections with long-term NAg (and Ag⁺) treatments of biofilm growth. The present study further observed a potential silver/antibiotic cross-resistance, whereby NAg (as well as Ag⁺) could not eradicate an already growing gentamicin-resistant P. aeruginosa biofilm. The phenomena is thought to result from the hindered biofilm penetration of the silver species. In contrast, both silver formulations inhibited biofilm formation of the resistant strain, presenting a promising avenue for the control of biofilm-forming antibiotic-resistant bacteria. Conclusion: The findings signify the importance to study the nanoparticle adaptation phenomena in the biofilm mode of bacterial growth, which are apparently unique to those already reported with the planktonic growth counterparts. This work sets the foundation for future studies in other globally significant bacterial pathogens when present as biofilms. Scientifically based strategies for management of pathogenic growth is necessary, particularly in this era of increasing antibiotic resistance. Graphic